Structural damage mechanisms associated with differential ground movements

Structural damage caused by differential ground movements has become a major concern in areas where mining or groundwater/oil extraction is carried out. Structural damage mechanisms associated with differential ground movements are investigated in this research. Twelve test foundations were constructed over a longwall mining panel. Different damage mitigation techniques were incorporated into each foundation. The damage mitigation techniques included three types of reinforcement (post-tensioning, rebar, and steel fiber) and two types of footing-soil interface (plastic, and plastic-on-sand). Each foundation was extensively instrumented. The foundation behavior and the movements of the nearby ground were also measured during the subsidence. The damaged test foundations were repaired, and the response of the repaired foundations and an adjacent residential structure were monitored during a second subsidence event.

The features of the measured ground movements were discussed and compared with other mining panels. It was found that the structural response to the mine subsidence was to a large extent dependent on the relative location of the structure with respect to the subsidence trough. The structures in the centerline area experienced tension-compression-tension deformation phases as the ground underwent tension-compression deformation and finally returned to level. The structures in the tension zone area sustained permanent tension. All proposed damage mitigation techniques helped to reduce damage to a certain degree, with the post-tensioning being most effective. Footings developed major cracks at about one-third of the footing length from the trailing edge of the footing. This suggests that a larger moment developed around one-third of the footing length. Larger concrete strains and curvatures were also measured around this section in the uncracked footings. Forces developed in the concrete footing during the subsidence consisted of bending moments and axial forces. Bending moments were the dominant cause of footing cracking. About 70% to 100% of the ground curvature was transferred to the structure during the tension phase while only about 30% to 50% of the ground deformation was transferred to the structure during the compression phase. For a typical low-rise, residential house, curvature of 0.0002 (1/m) was determined to be an appropriate threshold value corresponding to functional damage.